Instead of paying over the odds for fashionable foods, we would be better eating a balanced diet, rich in everyday fruit and vegetables, they say. Catherine Collins, chief dietician at St George's Hospital, in South London, said: "The term 'superfoods' is at best meaningless and at worst harmful.

"There are so many wrong ideas about superfoods that I don't know where to begin to dismantle the concept.

"Nominating some foods as nutritional talismans gives the impression that ordinary, affordable and everyday foods are somehow deficient.

"But rather than spend £5 on a

punnet of exotic berries, a family would be better buying regular and larger portions of fresh fruit and vegetables from their local market.

"On a restricted budget, it is even more important to ignore dubious, expensive products in the belief you can take short cuts to a good diet.

"Rather than buying some ridiculous African algae, with all the emissions associated with travel, eating a cheap British apple would be better for the environment too."

Mrs Collins, who is also a member of the British Dietetic Association, said that high levels of vitamins are not always beneficial for the body.

She said: "It might seem that eating foods rich in nutrients is just common sense, but the truth is that our bodies have a requirement for sufficient nutrients.

"If our bodies have an excess or nutrients and cannot store them, they will essentially go to waste.

"Or if certain nutrients can't be excreted in sufficient levels, they could cause serious cellular damage. Overloading our bodies is not a healthy or a natural thing to do."

Dr Jeremy Spencer, a Reading University nutritionist, said: "It is impossible to predict the reactions of individual metabolisms to specific foods."

He told the Observer: "People don't eat nutrients, they eat foods. And foods can behave very differently to the nutrients they contain and they can have a very different effect in someone's body than they have when examined in a test tube."

The dietary experts, who will take part in a superfoods debate at the Science Museum this week, said instead of obsessing over specific foods, we should eat a varied diet.

A Mediterranean diet, high in fruit, vegetables and nuts and low in saturated fat, could be of particular benefit.

Mrs Collins added: "People should not look for individual superfoods but should try to eat a 'super diet'."